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-rw-r--r--doc/intro.ur12
1 files changed, 8 insertions, 4 deletions
diff --git a/doc/intro.ur b/doc/intro.ur
index 02adfdd2..e3611567 100644
--- a/doc/intro.ur
+++ b/doc/intro.ur
@@ -259,20 +259,24 @@ signature STACK = sig
con t :: Type -> Type
val empty : a ::: Type -> t a
val push : a ::: Type -> t a -> a -> t a
- val pop : a ::: Type -> t a -> option a
+ val peek : a ::: Type -> t a -> option a
+ val pop : a ::: Type -> t a -> option (t a)
end
structure Stack : STACK = struct
con t = list
val empty [a] = []
fun push [a] (t : t a) (x : a) = x :: t
+ fun peek [a] (t : t a) = case t of
+ [] => None
+ | x :: _ => Some x
fun pop [a] (t : t a) = case t of
- [] => None
- | x :: _ => Some x
+ [] => None
+ | _ :: t' => Some t'
end
(* begin eval *)
-Stack.pop (Stack.push (Stack.push Stack.empty "A") "B")
+Stack.peek (Stack.push (Stack.push Stack.empty "A") "B")
(* end *)
(* Ur also inherits the ML concept of <b>functors</b>, which are functions from modules to modules. *)